# libATA Developer's Guide

Jeff
Garzik
2003-2005
Jeff Garzik

The contents of this file are subject to the Open Software License version 1.1 that can be found at http://www.opensource.org/licenses/osl-1.1.txt and is included herein by reference.

Alternatively, the contents of this file may be used under the terms of the GNU General Public License version 2 (the "GPL") as distributed in the kernel source COPYING file, in which case the provisions of the GPL are applicable instead of the above. If you wish to allow the use of your version of this file only under the terms of the GPL and not to allow others to use your version of this file under the OSL, indicate your decision by deleting the provisions above and replace them with the notice and other provisions required by the GPL. If you do not delete the provisions above, a recipient may use your version of this file under either the OSL or the GPL.

## Introduction

libATA is a library used inside the Linux kernel to support ATA host controllers and devices. libATA provides an ATA driver API, class transports for ATA and ATAPI devices, and SCSI<->ATA translation for ATA devices according to the T10 SAT specification.

This Guide documents the libATA driver API, library functions, library internals, and a couple sample ATA low-level drivers.

## libata Driver API

struct ata_port_operations is defined for every low-level libata hardware driver, and it controls how the low-level driver interfaces with the ATA and SCSI layers.

FIS-based drivers will hook into the system with ->qc_prep() and ->qc_issue() high-level hooks. Hardware which behaves in a manner similar to PCI IDE hardware may utilize several generic helpers, defining at a bare minimum the bus I/O addresses of the ATA shadow register blocks.

### struct ata_port_operations

#### Disable ATA port

    void (*port_disable) (struct ata_port *);

Called from ata_bus_probe() and ata_bus_reset() error paths, as well as when unregistering from the SCSI module (rmmod, hot unplug). This function should do whatever needs to be done to take the port out of use. In most cases, ata_port_disable() can be used as this hook.

Called from ata_bus_probe() on a failed probe. Called from ata_bus_reset() on a failed bus reset. Called from ata_scsi_release().

#### Post-IDENTIFY device configuration

    void (*dev_config) (struct ata_port *, struct ata_device *);

Called after IDENTIFY [PACKET] DEVICE is issued to each device found. Typically used to apply device-specific fixups prior to issue of SET FEATURES - XFER MODE, and prior to operation.

Called by ata_device_add() after ata_dev_identify() determines a device is present.

This entry may be specified as NULL in ata_port_operations.

#### Set PIO/DMA mode

    void (*set_piomode) (struct ata_port *, struct ata_device *);
    void (*set_dmamode) (struct ata_port *, struct ata_device *);
    void (*post_set_mode) (struct ata_port *ap);

Hooks called prior to the issue of SET FEATURES - XFER MODE command. dev->pio_mode is guaranteed to be valid when ->set_piomode() is called, and dev->dma_mode is guaranteed to be valid when ->set_dmamode() is called. ->post_set_mode() is called unconditionally, after the SET FEATURES - XFER MODE command completes successfully.

->set_piomode() is always called (if present), but ->set_dma_mode() is only called if DMA is possible.

#### Taskfile read/write

    void (*tf_load) (struct ata_port *ap, struct ata_taskfile *tf);
    void (*tf_read) (struct ata_port *ap, struct ata_taskfile *tf);

->tf_load() is called to load the given taskfile into hardware registers / DMA buffers. ->tf_read() is called to read the hardware registers / DMA buffers, to obtain the current set of taskfile register values. Most drivers for taskfile-based hardware (PIO or MMIO) use ata_tf_load() and ata_tf_read() for these hooks.

#### ATA command execute

    void (*exec_command)(struct ata_port *ap, struct ata_taskfile *tf);

causes an ATA command, previously loaded with ->tf_load(), to be initiated in hardware. Most drivers for taskfile-based hardware use ata_exec_command() for this hook.

#### Per-cmd ATAPI DMA capabilities filter

    int (*check_atapi_dma) (struct ata_queued_cmd *qc);

Allow low-level driver to filter ATA PACKET commands, returning a status indicating whether or not it is OK to use DMA for the supplied PACKET command.

This hook may be specified as NULL, in which case libata will assume that atapi dma can be supported.

#### Read specific ATA shadow registers

    u8   (*check_status)(struct ata_port *ap);
    u8   (*check_altstatus)(struct ata_port *ap);
    u8   (*check_err)(struct ata_port *ap);

Reads the Status/AltStatus/Error ATA shadow register from hardware. On some hardware, reading the Status register has the side effect of clearing the interrupt condition. Most drivers for taskfile-based hardware use ata_check_status() for this hook.

Note that because this is called from ata_device_add(), at least a dummy function that clears device interrupts must be provided for all drivers, even if the controller doesn't actually have a taskfile status register.

#### Select ATA device on bus

    void (*dev_select)(struct ata_port *ap, unsigned int device);

Issues the low-level hardware command(s) that causes one of N hardware devices to be considered 'selected' (active and available for use) on the ATA bus. This generally has no meaning on FIS-based devices.

Most drivers for taskfile-based hardware use ata_std_dev_select() for this hook. Controllers which do not support second drives on a port (such as SATA contollers) will use ata_noop_dev_select().

#### Reset ATA bus

    void (*phy_reset) (struct ata_port *ap);

The very first step in the probe phase. Actions vary depending on the bus type, typically. After waking up the device and probing for device presence (PATA and SATA), typically a soft reset (SRST) will be performed. Drivers typically use the helper functions ata_bus_reset() or sata_phy_reset() for this hook. Many SATA drivers use sata_phy_reset() or call it from within their own phy_reset() functions.

#### Control PCI IDE BMDMA engine

    void (*bmdma_setup) (struct ata_queued_cmd *qc);
    void (*bmdma_start) (struct ata_queued_cmd *qc);
    void (*bmdma_stop) (struct ata_port *ap);
    u8   (*bmdma_status) (struct ata_port *ap);

When setting up an IDE BMDMA transaction, these hooks arm (->bmdma_setup), fire (->bmdma_start), and halt (->bmdma_stop) the hardware's DMA engine. ->bmdma_status is used to read the standard PCI IDE DMA Status register.

These hooks are typically either no-ops, or simply not implemented, in FIS-based drivers.

Most legacy IDE drivers use ata_bmdma_setup() for the bmdma_setup() hook. ata_bmdma_setup() will write the pointer to the PRD table to the IDE PRD Table Address register, enable DMA in the DMA Command register, and call exec_command() to begin the transfer.

Most legacy IDE drivers use ata_bmdma_start() for the bmdma_start() hook. ata_bmdma_start() will write the ATA_DMA_START flag to the DMA Command register.

Many legacy IDE drivers use ata_bmdma_stop() for the bmdma_stop() hook. ata_bmdma_stop() clears the ATA_DMA_START flag in the DMA command register.

Many legacy IDE drivers use ata_bmdma_status() as the bmdma_status() hook.

#### High-level taskfile hooks

    void (*qc_prep) (struct ata_queued_cmd *qc);
    int (*qc_issue) (struct ata_queued_cmd *qc);

Higher-level hooks, these two hooks can potentially supercede several of the above taskfile/DMA engine hooks. ->qc_prep is called after the buffers have been DMA-mapped, and is typically used to populate the hardware's DMA scatter-gather table. Most drivers use the standard ata_qc_prep() helper function, but more advanced drivers roll their own.

->qc_issue is used to make a command active, once the hardware and S/G tables have been prepared. IDE BMDMA drivers use the helper function ata_qc_issue_prot() for taskfile protocol-based dispatch. More advanced drivers implement their own ->qc_issue.

ata_qc_issue_prot() calls ->tf_load(), ->bmdma_setup(), and ->bmdma_start() as necessary to initiate a transfer.

#### Timeout (error) handling

    void (*eng_timeout) (struct ata_port *ap);

This is a high level error handling function, called from the error handling thread, when a command times out. Most newer hardware will implement its own error handling code here. IDE BMDMA drivers may use the helper function ata_eng_timeout().

#### Hardware interrupt handling

    irqreturn_t (*irq_handler)(int, void *, struct pt_regs *);
    void (*irq_clear) (struct ata_port *);

->irq_handler is the interrupt handling routine registered with the system, by libata. ->irq_clear is called during probe just before the interrupt handler is registered, to be sure hardware is quiet.

The second argument, dev_instance, should be cast to a pointer to struct ata_host_set.

Most legacy IDE drivers use ata_interrupt() for the irq_handler hook, which scans all ports in the host_set, determines which queued command was active (if any), and calls ata_host_intr(ap,qc).

Most legacy IDE drivers use ata_bmdma_irq_clear() for the irq_clear() hook, which simply clears the interrupt and error flags in the DMA status register.

#### SATA phy read/write

    u32 (*scr_read) (struct ata_port *ap, unsigned int sc_reg);
    void (*scr_write) (struct ata_port *ap, unsigned int sc_reg,
                       u32 val);

Read and write standard SATA phy registers. Currently only used if ->phy_reset hook called the sata_phy_reset() helper function. sc_reg is one of SCR_STATUS, SCR_CONTROL, SCR_ERROR, or SCR_ACTIVE.

#### Init and shutdown

    int (*port_start) (struct ata_port *ap);
    void (*port_stop) (struct ata_port *ap);
    void (*host_stop) (struct ata_host_set *host_set);

->port_start() is called just after the data structures for each port are initialized. Typically this is used to alloc per-port DMA buffers / tables / rings, enable DMA engines, and similar tasks. Some drivers also use this entry point as a chance to allocate driver-private memory for ap->private_data.

Many drivers use ata_port_start() as this hook or call it from their own port_start() hooks. ata_port_start() allocates space for a legacy IDE PRD table and returns.

->port_stop() is called after ->host_stop(). It's sole function is to release DMA/memory resources, now that they are no longer actively being used. Many drivers also free driver-private data from port at this time.

Many drivers use ata_port_stop() as this hook, which frees the PRD table.

->host_stop() is called after all ->port_stop() calls have completed. The hook must finalize hardware shutdown, release DMA and other resources, etc. This hook may be specified as NULL, in which case it is not called.

## libata Library

!Edrivers/scsi/libata-core.c

## libata Core Internals

!Idrivers/scsi/libata-core.c

## libata SCSI translation/emulation

!Edrivers/scsi/libata-scsi.c !Idrivers/scsi/libata-scsi.c

## ata_piix Internals

!Idrivers/scsi/ata_piix.c

## sata_sil Internals

!Idrivers/scsi/sata_sil.c

## Thanks

The bulk of the ATA knowledge comes thanks to long conversations with Andre Hedrick (www.linux-ide.org), and long hours pondering the ATA and SCSI specifications.

Thanks to Alan Cox for pointing out similarities between SATA and SCSI, and in general for motivation to hack on libata.

libata's device detection method, ata_pio_devchk, and in general all the early probing was based on extensive study of Hale Landis's probe/reset code in his ATADRVR driver (www.ata-atapi.com).